I. Field of the Invention
The present invention is directed generally to dust filtration systems for separating entrained particulate material from a conveying atmosphere or other gaseous fluid escaping a vessel. More particularly, the invention is directed to an improved plenum pulsed filter vent system for use in association with cement silos or cement batching vessels associated with transportable or stationary concrete batching plants or other containers or vessels subject to the generation of relatively large amounts of dust or other particulate matter internally during use.
II. Related Art
Plants for the storage and blending of the ingredients of concrete, particularly Portland cement, present a prime example of an application for the plenum pulsed filter vent system of the invention. Permanent plants for preparing batches of concrete, by mixing stone, sand, Portland cement and water, have long been used for pre-mixing materials in a weigh batching vessel prior to transferring them to transit carriers, including transit concrete mixer trucks, for delivery to job sites for use. Mixed concrete, of course, is quite heavy and has a very limited open time, making it difficult to transport over long distances from permanent mixing plant installations to remote job sites, such as found in highway construction or the building of other facilities requiring large amounts of concrete.
This situation has been alleviated by the development of transportable concrete batching systems which are capable of being moved over highways to be set up close to the point of then current use. These systems are knock-down or readily disassembled units which fold and unfold on themselves to a certain degree for transport and installation at a desired batching site. The required components of the mix, which have no limited shelf life, can then be transported to the batching facility over longer distances as needed and loaded into containers associated with the portable batching plant for later dispensing and/or mixing in the desired proportions as truck-load batches in a weigh batching vessel.
One example of such a transportable concrete batching apparatus is illustrated and described in U.S. Pat. No. 5,121,989 assigned to the same assignee as the present application. That patent is deemed incorporated herein by reference for any purpose.
A main ingredient in any case, of course, is Portland cement and the containers include a relatively large cement storage silo. Portland cement is a finely divided particulate material which produces a rather large amount of fine airborne dust during the charging operation when the silo or weigh batching vessel is refilled. This necessitates the provision of a filter vent system associated with the venting of air displaced from the cement silo, particularly during loading operations when almost the entire volume is vented as the silo is recharged. The same is true in the loading of dry ingredients including Portland cement into a batching vessel.
The large volume of airborne dust generated within the cement-containing vessels, particularly during filling operations, makes it important that the vent filters in these facilities deal with and/or prevent the escape of rather large amounts of fine entrained airborne particulate matter. This material encounters and quickly covers upstream filter surfaces and clogging of the filters is a persistent problem. Dust collection systems associated with cement silos; for example, typically include an open-bottomed chamber provided with a top panel from which one or more substantially vertically positioned hollow filter cartridge elements are mounted. The outside or upstream side of each filter element is exposed to the silo interior volume and the inside or downstream side is exposed to a clean air plenum which connects to the vent opening. In this manner, air escaping from the cement silo must traverse a cartridge filter element prior to escaping from a vent in the clean air plenum above the filter system. In order to maintain air flow through the filter system, it is necessary to provide a system to repeatedly clean or unclog the filter cartridges during loading of the silo when large amounts of air are being displaced.
Systems periodically injecting pulses of air, have been used in the past to clean the filters. These systems have relied on multiple diaphragm valves, pulse pipes and nozzles for cleaning. In such systems, a nozzle is located so as to blast air locally into an adjacent filter cartridge. This requires that a separate nozzle be provided for every cartridge or group of cartridges. Another drawback with such systems is that if the nozzles are not located at the correct distance above the filter, air from the pulse will not enter the filter correctly and the filter will not be adequately cleaned. For example, if the nozzles are located too close to the filter opening (too low), the pulse will affect cleaning only in the lower part of the filter and the top of the filter will not adequately be cleaned. If, on the other hand, the nozzles are located too far above the filters, the full force of the pulses will not enter the filters at all and the bottom of the filters will not be adequately cleaned.
In addition, the height requirement of many present pulse, pipe and nozzle filter cleaning systems is so great that maintenance personnel must climb a ladder to get to the top of the filter vent to load and unload the cartridges. This may present a safety concern inasmuch as filter vents are on top of silos that may be as high as 60 feet or more above the ground. Some of these systems are provided with a side entry to be utilized to replace the cartridges. In these designs, because the venting system is mounted in a rather large open-bottom plenum on the top of the silo and since maintenance personnel must reach into or enter the vent filtering system to change cartridges, OSHA enclosed space rules apply. Maintenance personnel therefore may be required to work in pairs and wear special body harnesses or other safety gear to comply with OSHA rules when changing cartridges. This, of course, represents added cost and results in an undesirable situation.
There clearly remains a need to provide a plenum filter vent system which assures rapid periodic cleaning of the entire filter cartridges and which, at the same time, includes a construction design which facilitates the changing of the cartridges when this operation is necessary.
The present invention provides a filter vent system and method for separating entrained airborne particulate matter from the conveying fluid particularly as it relates to filling a storage vessel of interest. The filter system employs a simplified, yet more efficient filter cleaning arrangement, in which the entire clean air plenum on the downstream side of a filter array is periodically pulsed with a large burst or pulse of relatively high pressure air from a single source which rapidly raises the plenum pressure to, in turn, produce a temporary pulsed flow reversal in the filter units which blasts through the filter pores to displace particulate matter collected on the upstream side of all of the filter units alike thereby cleaning or re-generating the filter surfaces.
This plenum pulsing operation is repeated on a predetermined timed basis during the vessel charging operation and for a scheduled time thereafter. Generally, but not by way of limitation, pulse air is supplied at a pressure ranging from about 60 to 100 PSI and preferably at or above 75 PSI. Pulse duration may be varied as desired but a range of from about 50 to about 150 ms and preferably 80 to 100 ms has proved effective. The interval between pulses may also be varied but arranged between about 60 and 180 seconds is generally used with 120 seconds being an average time. The volume of pulsed air or size of the pulse supply pipe or the like will vary with plenum chamber size and number of elements.
A pressure measurement and switch system for measuring the pressure drop across the filter units and operating a switch or other device based on pressure loss can also be used to provide data or control the operation of the plenum pulsing system. In addition, an exhaust blower may be provided to assist in exhausting the clean air plenum of a vessel being charged rapidly with solid material such as Portland cement. As indicated, Plenum and filter volume, together with pulse pipe diameter and storage vessel size considerations enter into the determination of optimum pulse parameters for any particular system.